Ink-jet printing is a non-impact printing process in which droplets of ink are deposited on a print medium in a particular order to form alphanumeric characters, area fills, and other patterns thereon. Low cost and high quality of the output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers.
The non-impact printing process of ink-jet printing involves the ejection of fine droplets of ink onto a print media such as paper, transparency and opaque film, or textiles in response to electrical signals generated by a microprocessor. There are two basic means currently available for achieving ink droplet ejection ink-jet printing: thermally and piezoelectrically. In thermal ink-jet printing, the energy for drop ejection is generated by electrically heated resistor elements, which heat up rapidly in response to electrical signals from a microprocessor to create a vapor bubble, resulting in the expulsion of ink through nozzles associated with the resistor elements. In piezoelectric ink-jet printing, the ink droplets are ejected due to the vibrations of piezoelectric crystals, again, in response to electrical signals generated by the microprocessor.
In commercially-available thermal ink-jet color printers, such as a DeskJet.RTM. printer available from Hewlett-Packard Company, a color spectrum is achieved by combining yellow, magenta, cyan, and black inks in various proportions. The yellow, magenta, cyan, and black inks derive their hues from yellow, magenta, cyan, and black colorants, respectively.
Colorants for inks are available in the form of dyes or pigments. Accordingly, ink-jet inks are available as dye-based and/or pigment-based compositions. Of the two, dye-based ink-jet ink compositions are much more widely available. Dye-based ink-jet ink compositions are generally aqueous-based and are formulated by dissolving dye in an ink vehicle. The dye molecules employed in ink-jet ink compositions are often in the form of dye salts made of a dye anion and a cation such as sodium, lithium, or tetramethylammonium (TMA). A limited number of pigment-based ink-jet inks are also available, which generally comprise a pigment dispersed in an aqueous solution by a dispersant. Although pigments offer the very desirable properties of waterfastness and lightfastness, their natural tendency to agglomerate in aqueous media and their lack of uniform size distribution have dampened industry enthusiasm for their employment as ink-jet ink compositions in comparison to dyebased inks.
Good print quality in ink-jet printing is quantified by several different measuring sticks. For example, three desirable qualities in ink-jet ink compositions are waterfastness, lightfastness, and good edge acuity. In addition to these measures of good print quality, ink-jet ink compositions are subject to an additional measuring stick when printed on transparencies or glossy media, both film- and paper-based: clarity. More specifically, it is desirable that an image on a transparency or glossy media be free of cloudiness or haziness, which would naturally go unnoticed for images printed onto non-glossy paper media.
One likely source for undesirable cloudiness or haziness associated with images printed on transparencies or glossy media is crystallization of dye molecules. Such crystallization has been linked to yellow dye-based ink-jet ink compositions containing the anionic dye molecule Acid Yellow 23. For inks containing Acid Yellow 23, a cloudiness or haziness may be observed to develop over time after an image is printed onto a transparency or glossy media. The crystallization of the Acid Yellow 23 dye molecule increases light scattering on transparencies and glossy media, both film- and paper-based, producing a yellow-brown appearance. Moreover, this cloudiness may be observed in any inked areas containing the yellow ink; thus, yellow, red, and green areas each potentially display the problem.
Thus, a need remains for a solution to the problem of crystallization of Acid Yellow 23 anionic dye molecules such that images produced on transparencies and glossy media (both film- and paper-based) with such dye molecules are lucid rather than cloudy or hazy. The solution to the problem of crystallization of Acid Yellow 23 anionic dye molecules should be effective, easily implemented, and inexpensive. Finally, overall print quality should not be sacrificed.